IMRT In Breast Cancer

Worldwide breast cancer is the most common malignancy in women. In the developed world, it is responsible for 18 percent of all cases of cancers seen in women. One million new cases of breast cancer are registered worldwide every year and it is the single commonest cause of death among women in the 40- 50 years age group. In India breast cancer is the second commonest cancer seen in the women after carcinoma cervix. Its prevalence is higher in urban women and it accounts for 20% of all cancer related diseases. In Mumbai and Delhi, it is the commonest malignancy seen in women. In India it has an incidence of 17-40 cases per 1, 00,000 population and the 5 years survival is 42.3 percent to 46.8 percent.

Conventionally radiotherapy in early breast cancers is done by the whole breast technique which utilizes two tangential ports. Upper margin of the radiotherapy field lies at the first intercostals space, and the lower margin lies 2 cm below the inferior mammary line. Medial margin is in the midsternal line and the lateral margin is at the mid axillary line. A total dose of 50 Gy in 25 fractions is given over 5 weeks by conventional fractionation followed by boost of 10-20 Gy. This technique is easy to setup, and avoids the junction dose. Computerizes planning is done for dose optimization. Standard 2d treatment (conventional radiotherapy) techniques utilizes a simplistic view of patient anatomy and it creates ‘hot spot’ because lung transmission is not accurately included and thinner regions of breast (superior and inferior) are modeled like the thickest slice of the breast. Thus conventional radiotherapy has limitations in the treatment of breast cancer and these include.

• Dose inhomogeniety – due to change in the contours of the breast 15–20 percent of dose inhomogenicity in the superior and the inferior planes of the breast occurs. The medial and lateral aspects of the breast get higher doses of radiation.

• Radiation accompaniments i. e. radiation effects on normal tissues in the field are seen uncommonly but they do occur in the lungs and the heart. Newer techniques are able to minimize them.

The newer techniques are used to

• Improve dose homogeneity within the tumor volume.
• Avoid radiation to normal tissues in the area.
• Reduce side effects related to the radiation treatment.
• Improve local tumor control and overall survival of the patient.
In an attempt to address the above mentioned parameters a number of newer radiotherapy techniques have been introduced for the treatment of early breast cancer. These include IMRT, external beam RT using 3D conformal RT, Intraoperative Electron Beam RT, Mammosite Ballon Branchytherapy and interstitial branchytherapy. Gated radiotherapy is also available.

All these techniques need a CT scan based treatment planning system and require the use of tissue compensators.

IMRT

IMRT is an approach to conformal therapy that not only delivers high dose to the tumor tissue but also ensures low dose to the surrounding normal tissue. The dose is varied depending on the tumor volume. A higher dose of radiation can be delivered to the areas with high tumor volume, a small dose where tumor volume is not so high and a minimal dose is delivered to surrounding normal tissue. By these means a higher tumor control probability and minimal or no side effects of radiotherapy are achieved, resulting in improved therapeutic ratio and better patient care.

Important normal structures that need to be examined and protected while IMRT for breast cancer is being planned include

• Heart
• Ipsilateral lung
• Contralateral lung
• Contralateral breast
• Tissues outside the breast planned tumor volume (PTV)
Dose specifications in IMRT are as follows:

• Breast volume receiving 105% of prescribed dose should be < 15%
• Breast volume receiving 110% of prescribed dose should be 55
• Breast receiving 115% of prescribed dose should be <1%

STANDARD IMRT TECHNIQUE

It is two field IMRT and two tangent beam likely to have sharp dose gradients at edges of beam. This spares the normal structures in the surroundings area so treatment toxicity decreases. Also there is better tumor coverage and hence increased tumor control.

FIELD IN THE FIELD TECHNIQUE (POOR MAN’S IMRT)

In this technique whole breast radiation is given simultaneously with a boost to the involved quadrant. It is very time consuming and therefore limits the number of patients each planner can treat. The planning time and quality of plan is highly dependent upon the skill level of the treatment planner. Its utility is therefore questionable as the variables are not easily controlled.

HYBRID IMRT

The combines conventional beams RT wit IMRT beams to get soft dose gradients at the edge of the beam and good dose conformality within the irradiation area. This results in almost negligible side effects.

When breast cancer is treated with IMRT there is considerable reduction in the dose to contralateral breast. The dose to the contralateral breast depends on
• Wedge angle used
• Type of wedge used
• Shape of contralateral breast
• Gantry angle
• Type of technique used for treatment

GATED RADIOTHERAPY is a technique in which motion of the tumor due to breathing and other reasons is taken into account while radiation is being delivered. This is very useful for the treatment of lung, chest and abdominal malignancies.

To summarize IMRT in the breast cancer results in maximum radiation dose to the tumor resulting in better tumor control and minimal side effects to the lungs and heart. It is more useful for left sided lesions as compared with tumours that occur in the right side.

Author on various topics for Cancer, Radiation Oncology, Allied Services, Healthcare, Hospitals and Health Subjects.
For more information you can visit Cancer Hospital India

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